This invention relates generally to radio frequency (RF) power amplifiers, and more particularly the invention relates to an RF power amplifier field effect transistor (FET) suitable for communications networks where low intermodulation distortion (IMD) products are a key design parameter.
Impedance matching is an important criteria of amplifier design and performance. Optimum gain is realized when the maximum available source power is delivered to the amplifier and the maximum available output power is extracted from the amplifier.
The impedance matching topography may take on a number of forms. Low-pass, high-pass, and band-pass techniques are typically employed using various combinations of lumped and distributed components. However, one dominant feature consistent throughout all prior designs is the impedance seen at the transistor terminals is simplified to an easily manageable two-port network. The two-port network design might be characterized by S-parameters, Y-parameters, Z-parameters, or any of the other known network design parameters.
FIG. 1 shows a typical low-pass matching topology that has been used to match a field effect transistor (FET) amplifier. The FET model, T1, includes a common-source input capacitance (Ciss), a common-source output capacitance (Coss), and a common-source capacitance between the gate and drain (Crss). An input resistor, Z.sub.s, is equal to the input resistance of the gate and source when the field effect transistor is conducting. In accordance with prior art design techniques, a designer merely presents a conjugate impedance to the respective terminal once the terminal impedance has been appropriately characterized. For example, the input inductor, L1, in FIG. 1 matches the equivalent amplifier input impedance to the source impedance. Similarly, the output inductor, L2, matches the equivalent amplifier output impedance to the load impedance.
While the conjugate-matching technique will optimize the amplifier power gain, intermodulation distortion (IMD) products can be unpredictable. Intermodulation distortion is characterized by the appearance in the output of frequencies equal to the sums and differences of integral multiples of the two or more component frequencies present in an input wave. Most often, the amplifier must be mismatched at the output terminals to minimize unwanted IMD products. The tradeoffs between gain, efficiency and spectral purity compromise the optimum design.
The present invention is directed to providing a high gain, low IMD product, high efficiency RF power amplifier.